The present invention pertains generally to systems for levitating and propelling a magnetically levitated (MAGLEV) vehicle. More particularly, the present invention pertains to levitation and propulsion systems that are energy efficient over a pre-selected range of vehicle speeds. The present invention is particularly, but not exclusively, useful as an efficient levitation and propulsion system consisting of a linear synchronous motor (LSM) and an Electro-Dynamic (levitation) System (EDS) arranged uniquely into an inherently stable system.
Magnetic levitation systems, often called MAGLEV systems, use magnetic fields to levitate a vehicle over a stationary guideway. Because the vehicle does not physically contact the guideway during acceleration and normal high-speed operation, energy losses associated with contact friction are greatly reduced. Still, magnetic drag forces, if not taken into consideration, can offset the benefits of reduced contact friction resulting in an inefficient transportation system.
Heretofore, electromagnetic systems (EMS) and Electro-Dynamic Systems (EDS) have been used to levitate vehicles. The EMS systems use the attraction between electromagnets attached to the vehicle and iron rails on the guideway to produce the required levitation force. Such a system is inherently unstable and active control of the electromagnets is required to maintain the gap between the iron rails and the electromagnets. Specifically, any fluctuation in the gap from irregularities in the track or external forces on the vehicle/guideway must be immediately countered. As one might expect, the required active control system is complicated, expensive and unreliable. With the Electro-Dynamic System (EDS), eddy currents induced in electrically conductive material on the track by a traveling magnetic field from magnets on the vehicle, produce a levitation force by their interaction with the array of magnets in the vehicle. This interaction produces drag forces that must be overcome by the propulsion system.
A linear synchronous motors (LSM), generates forces that can be used to propel a vehicle and additionally, forces that act in a direction orthogonal to the direction of propulsion. One example of a linear synchronous motor includes an armature having an a.c. poly-phase winding on an armature. This armature can be mounted on the stationary guideway for interaction with permanent magnets mounted on the vehicle. By embedding the armature winding in ferromagnetic material, attractive forces generated between the armature and the magnets can be used to levitate the vehicle. These attractive forces are generated without also generating drag forces.
Alone, the LSM is unstable and the LSM gap between the armature and magnets cannot be maintained. Specifically, even slight decreases in the LSM gap cause the attractive force between the armature and magnets to increase, and the increased force acts to further close the LSM gap. In addition to stability considerations, the efficiency of the linear synchronous motor must be considered. In this regard, the efficiency of the LSM is highly dependant on the width of the LSM gap. Specifically, the LSM is most efficient when the LSM gap is maintained relatively small.
In light of the above, it is an object of the present invention to provide systems suitable for the purposes of levitating and propelling a vehicle over a guideway that are stable and energy efficient. It is another object of the present invention to provide MAGLEV levitation and propulsion systems that provide passive levitation control with reduced peak and average magnetic drag forces. It is yet another object of the present invention to provide MAGLEV levitation and propulsion systems that remain stable in spite of unwanted fluctuations in propulsion system currents and/or external forces acting on the vehicle. Still another object of the present invention is to provide MAGLEV levitation and propulsion systems that can be used to provide lateral stability to the MAGLEV vehicle. It is still another object of the present invention to provide MAGLEV levitation and propulsion systems that are efficient during acceleration from low vehicle speeds (i.e. at peak power). It is another object of the present invention to provide MAGLEV levitation and propulsion systems that are efficient at high vehicle speeds (i.e. operating speeds). Yet another object of the present invention is to provide MAGLEV levitation and propulsion systems which are easy to use, relatively simple to implement, and comparatively cost effective.
The present invention is directed to a system for levitating and propelling a vehicle along a stationary guideway. In functional overview, the system is designed for energy efficiency over a predetermined vehicle speed range. In one embodiment, the system is designed for maximum power and efficiency during vehicle acceleration from zero speed. In another embodiment of the present invention, the system is designed for maximum efficiency at operational speeds. In addition to efficiency considerations, the system is designed to be inherently stable (unlike an EMS system or LSM system acting alone) in spite of increases in LSM armature winding current or external forces acting on the vehicle. This inherent stability of the system also allows the system to be used to provide lateral stability to the vehicle.
In accordance with the present invention, the levitation and propulsion system includes a linear synchronous motor (LSM) having two LSM components. One LSM component is mounted on the vehicle and the other LSM component is mounted on the guideway. When the vehicle is positioned on the guideway, the LSM components of the linear synchronous motor are juxtaposed and define an LSM gap between the LSM components.
For the present invention, one of the LSM components includes either a switched direct-current winding or a poly-phase winding on an iron core, and the other LSM component includes a plurality of magnetic poles mounted on a rail. Functionally, the linear synchronous motor is provided to produce a first electromagnetic force between the LSM components that acts to levitate the vehicle and a second electromagnetic force between the LSM components that acts to propel the vehicle along the guideway. Importantly, the magnitudes of these electromagnetic forces are dependent on the width of the LSM gap, the LSM armature winding current, the size of the iron core and the total vehicle load. As indicated above, a linear synchronous motor, by itself, is unstable and this instability closes the LSM gap prohibiting movement of the vehicle.
For the present invention, the levitation and propulsion system includes an electrodynamic system (EDS) to maintain the LSM gap within a desired width range. More specifically, a small LSM gap is maintained by the EDS over a predetermined range of vehicle speeds because the linear synchronous motor is most efficient when the LSM gap is small. Also, by maintaining the LSM gap within a desired width range, the LSM instabilities described above are eliminated.
Structurally, the electrodynamic system has an EDS component mounted on the vehicle and another EDS component mounted on the stationary guideway. During vehicle movement along the guideway, the EDS components cooperate to create an electromagnetic force that reacts with the levitation forces created by the LSM. Like the LSM, when the vehicle is positioned on the guideway, the EDS components are juxtaposed and define an EDS gap between the EDS components. In the preferred embodiment of the present invention, one of the EDS components is a magnet array and the other EDS component is a plurality of conductive cables, with each cable extending in a direction orthogonal to the direction of vehicle travel and short-circuited at both ends. Importantly, the magnitude of the electromagnetic force generated by the EDS is dependent on the width of the EDS gap and the speed of the vehicle relative to the stationary guideway.
As indicated above, the linear synchronous motor defines an LSM gap and the electrodynamic system defines an EDS gap. For the present invention, the vehicle is configured relative to the guideway to cause the width of these gaps to vary equally as the amount of vehicle levitation varies. In a first embodiment of the present invention, the widths of both the LSM and EDS gaps decrease with increasing vehicle levitation over the guideway.
In this embodiment, the LSM establishes an electromagnetic force that tends to levitate the vehicle while the EDS establishes an electromagnetic force that opposes levitation of the vehicle. For a vehicle at constant speed any change in levitation will change both the LSM and EDS gaps equally. Thus any change in levitation force generated by the LSM will be counteracted by a change in the EDS-generated force opposing levitation. By properly sizing the EDS and LSM systems, a substantially constant levitating force at constant speed can be obtained that results in stable vehicle travel.
For this embodiment, the opposing force generated by the EDS is small at low vehicle speeds. Since the opposing force is small, vehicle levitation is large and the LSM gap is small (See above). During acceleration, which is generally required when the vehicle is at low speeds, the LSM is most efficient with a small gap width. Thus, this embodiment of the present invention which maintains a small LSM gap width at low vehicle speeds, establishes an efficient levitation and propulsion system.
In an alternate embodiment of the present invention, the vehicle is configured wherein the width of the LSM gap decreases with increasing vehicle levitation over the guideway, while the width of the EDS gap increases with increasing vehicle levitation over the guideway. For this embodiment, both the LSM and the EDS establish electromagnetic forces that act to levitate the vehicle (i.e. no opposing force is created). For a vehicle at low speed and low vehicle levitation, the LSM gap is relatively large and the EDS gap is relatively small. Accordingly, both the LSM and EDS levitating forces are relatively weak. On the other hand, at higher vehicle speeds and levitations, the LSM gap is relatively small and the EDS gap is relatively large. Accordingly, at higher vehicle levitations the LSM levitating force is relatively strong and the EDS levitating force is relatively weak.
Like the first embodiment described above, in this embodiment, the EDS force acts with the LSM force to establish a substantially constant force over a range of LSM gap widths. This embodiment differs from the above described embodiment, however, because in this embodiment, maximum LSM efficiency is obtained at high vehicle speeds (i.e. operating speed) rather than during acceleration from low speed. In greater detail, at operating speed, the EDS force is relatively strong, the LSM levitating force is relatively weak, and the LSM gap is small. Thus, since the LSM is efficient at small LSM gap widths, in this embodiment, the system is efficient at operating speed.